1. Technical Field
The invention relates to an imaging lens system for use in an endoscope for examining the interior of the body, and more specifically, to an imaging lens system for use in a capsule type endoscope that is used in a manner that a patient swallows it.
2. Related Art
Diagnosis using an endoscope has been actively performed in the medical field. The diagnosis using an endoscope is performed by inserting, into the patient's body, a flexible insertion portion which is configured so that the flexible insertion portion can be operated freely to bend in a desirable direction. A small-size imaging portion including an illuminator, a imaging lens system, and an imaging device is provided at the end of the insertion portion. Therefore, the insertion type endoscope is arbitrarily changed within a predetermined range in the patient's body by the doctor's operation, and can accurately image a part which is suspected as being a lesion, which helps accurate medical diagnosis such as correct diagnosis and early detection of lesions.
Although the shape of the insertion-type endoscope is designed so as to reduce the load on the patient at the time of insertion, the insertion of the long insertion portion into the body is a heavy load on the patient. There may be cases where anesthesia or the like is necessary when the insertion-type endoscope is used.
In recent years, to further reduce the load on the patient, a so-called capsule type endoscope (hereinafter, may be referred to as a “capsule endoscope”) in which a small-size imaging apparatus is accommodated in a swallowable capsule has been being developed and is putting to practical use. Since this capsule endoscope is formed in an easily swallowable size, the load on the patient is significantly light as compared with the insertion-type endoscope. However, it is difficult to control its position and its direction inside the body.
Therefore, in order to take images of lesions without missing any even when the posture of the capsule endoscope inside the body is not desirable, it is desirable that the capsule endoscope be provided with a wide-angle imaging lens system so that an image of as wide area as possible can be taken.
From the above circumstances, a capsule endoscope has been known in which the capsule size is reduced while a wide imaging area is secured (see JP 2005-80789 A (corresponding to US 2005/0054902 A)). The following are also known: a capsule endoscope that attains not only a smaller size and a wider angle but also, for example, an optimum depth of field (see JP 2005-80790 A (corresponding to US 2005/0054901 A)); and a capsule endoscope that excellently illuminates a cylindrical structure (see JP 2006-61438 A (corresponding to US 2005/0124858 A, US 2007/0055105 A, and US 2008/0146877 A)).
It is important for the capsule endoscope to be easy to swallow. Therefore, in order to minimize its size, the capsule endoscope of the related art is provided with a lens including a minimum number of lenses, for example, a two-group, two-lens wide-angle lens as an imaging lens system.
Although it is desirable that the capsule endoscope be provided with a wide-angle imaging lens system as described above, more conspicuous distortion occurs as an imaging lens system has a wider angle of view like the other types of imaging lens systems. For example, as is often seen in wide-angle lenses, if a barrel-shaped distortion is caused, not only an obtained image is distorted but also the obtained image becomes small in the peripheral part of the obtained image.
Images taken by the capsule endoscope are images of the interior of the human body, and are images used for determining whether a lesion is present or absent. However, a lesion size differs according to a type and/or a degree of progression. In many cases, lesions are extremely small. Therefore, if such a small lesion is present in the peripheral part of the obtained image where a barrel-shaped distortion occurs as described above, the lesion would be smaller than its actual size and might be overlooked.
On the other hand, it is important for the capsule endoscope to be easy to swallow, and the capsule endoscope is used only once and then thrown away. Therefore, for price reduction and size reduction, the capsule endoscope is provided with a lens having a minimum number of lenses, for example, a two-group, two-lens wide-angle lens as its imaging lens system. There is no conventional imaging lens system for the capsule endoscope in which distortion is improved. Thus, an imaging lens system is demanded in which distortion is improved while its capsule size is kept small.